1. Field of the Invention
The present invention relates, in general, to presenilin 2 proteolytic fragments. In particular, the present invention relates to a purified 20 kDa presenilin 2 C-terminal fragment (PS2-CTF); purified nucleic acid molecules coding for the 20 kDa PS2-CTF protein; cells containing the nucleic acid molecules; non-human organisms containing the nucleic acid molecule; antibodies having specific binding affinity to the 20 kDa PS2-CTF; hybridomas containing the antibodies; methods of detecting 20 kDa PS2-CTF in a sample; diagnostic kits; methods for screening compounds that inhibit proteolytic processing of presenilin 2 in a cell, isolated compounds that inhibit proteolytic processing of presenilin 2 in a cell, and methods of inhibiting apoptotic cell death by preventing proteolytic cleavage of presenilin 2 at a cleavage site which generates a 20 kDa C-terminal fragment.
2. Related Art
A significant portion of Alzheimer's disease (AD) is attributed to specific gene defects leading to familial Alzheimer's disease (FAD) (Wasco, W., and Tanzi, R. E., in Molecular Genetics of Amyloid and Apolipoprotein E in Alzheimer's Disease (Dawbard, D., and Allen, B. J., eds), BIOS Scientific, Oxford, UK, (1995), pp. 51-76; Schallenberg, G. D., Proc. Natl. Acad. Sci. USA 92:8552-8559 (1995); Tanzi, R. E., el al., Neurobiol. Dis. 3:159-168 (1996); Sherrington, R., et al., Nature, 375:754-760 (1995); Levy-Lahad, E., et al., Science 269:973-977 (1995)). Two homologous genes; presenilin 1 (PS1) and presenilin 2 (PS2), are responsible for at least 50% of early onset (<60 years old) FAD (Schallenberg, G. D., Proc. Natl. Acad. Sci. USA 92:8552-8559 (1995); Tanzi, R. E., et al., Neurobiol. Dis. 3:159-168 (1996)). PS1 and PS2 are serpentine proteins consisting of six to nine predicted transmembrane domains interspersed with one large and multiple smaller hydrophilic loops (Sherrington, R., et al., Nature, 375:754-760 (1995); Levy-Lahad, E., et al., Science 269:973-977 (1995)). At the amino acid level, the two proteins are 67% identical and exhibit significant homology to two Caenorhabditis elegans gene products, sel-12 (approximately 50% identity) which has been predicted to facilitate Notch receptor function (Levitan, D., and Greenwald, I., Nature 377:351-354 (1995)), and spe-4 (approximately 26% identity) which is involved in cytoplasmic trafficking of proteins during spermatogenesis (L'Hernault, S. W., and Arduengo, P. M., J. Cell Biol. 119:55-68 (1992)).
PS1 and PS2 are ubiquitously expressed (Sherrington, R., et al., Nature, 375:754-760 (1995); Levy-Lahad, E., et al., Science 269:973-977 (1995)) and in brain are expressed primarily in neurons, with similar regional distributions (Kovacs, D. M., et al., Nat. Med. 2:224-229 (1996); Lee, M. K., et al., J. Neurosci. 16:7513-7525 (1996); Page, K., et al., Proc. Natl. Acad. Sci. USA 93:14020-14024 (1996)). The presenilins are localized to the endoplasmic reticulum (ER) and the Golgi apparatus but not the plasma membrane suggesting a potential role in protein processing (Kovacs, D. M., et al., Nat. Med. 2:224-229 (1996); Cook, D. G., Proc. Natl. Acad. Sci. USA 93:9223-9228 (1996); Walter, J., et al., Mol. Med. 2:673-691 (1996)). To date, the PS1 and PS2 genes have been shown to contain 35 different mutations which are inherited in an autosomal dominant fashion in over 60 kindreds with early onset FAD (Sherrington, R., et al., Nature, 375:754-760 (1995); Levy-Lahad, E., el al., Science 269:973-977 (1995); Rogasv, L. L., et al., Nature 376:775-778 (1995); for summary, see Kovacs, D. M., et al., Nat. Med. 2:224-229 (1996)). Recent studies suggest that the presenilins may directly or indirectly affect the processing of amyloid P-protein precursor, APP, leading to increased production of AP42 (Scheuner, D., et al., Nat. Med. 2:864-870 (1995); Duff, K., et al., Nature 383:710-713 (1996); Borchalt, D. R., et al., Neuron 17:1005-1013 (1995); Citron, M., et al., Nat. Med. 3:67-72 (1996)). These results help to explain the relatively high degree of amyloid burden in the brains of FAD patients carrying PS1 and PS2 mutations. The pathogenic mechanism by which presenilin mutations lead to increased P-amyloid deposition and other neuropathological features of AD remains unclear. To understand the role(s) of PS2 in normal cellular metabolism and AD pathogenesis, the processing and degradation pathways of PS2 was investigated.